Composite solid electrolyte

ABSTRACT

A composite solid electrolyte with excellent formability and chemical stability and high lithium ion conductivity. The composite solid electrolyte may comprise an oxide-based solid electrolyte and a sulfide-based solid electrolyte, wherein the oxide-based solid electrolyte is (Li 7−3Y−Z , Al Y )(La 3 )(Zr 2−Z , M Z )O 12  (where M is at least one element selected from the group consisting of Nb and Ta; Y is a number in a range of 0≦Y&lt;0.22; and Z is a number in a range of 0≦Z≦2), and wherein the sulfide-based solid electrolyte is VLiX—(1−V)((1−W)Li 2 S—WP 2 S 5 ) (where X is a halogen element; V is a number in a range of 0&lt;V&lt;1; and W is a number in a range of 0.125≦W≦0.30).

TECHNICAL FIELD

This disclosure relates to a composite solid electrolyte.

BACKGROUND

In the field of all-solid-state batteries, there has been an attempt toimprove the performance of all-solid-state batteries, focusing on aninterface between an electrode active material and a solid electrolytematerial.

For example, a solid electrolyte is disclosed in Patent Literature 1,which comprises a sulfide solid electrolyte with excellent formabilityat room temperature and an oxide solid electrolyte with excellentchemical stability.

Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No.2011-081915

Patent Literature 2: JP-A No. 2014-089971

Patent Literature 3: International Publication No. WO2012-176808

SUMMARY

However, prior-art composite solid electrolytes as disclosed in PatentLiterature 1 have a problem of very low lithium ion conductivity.

The disclosed embodiments were achieved in light of the abovecircumstance. An object of the disclosed embodiments is to provide acomposite solid electrolyte with excellent formability and chemicalstability and high lithium ion conductivity.

In a first embodiment, there is provided a composite solid electrolytecomprising an oxide-based solid electrolyte and a sulfide-based solidelectrolyte, wherein the oxide-based solid electrolyte is (Li_(7−3Y−Z),Al_(Y))(La₃)(Zr_(2−Z), M_(Z))O₁₂ (where M is at least one elementselected from the group consisting of Nb and Ta; Y is a number in arange of 0≦Y<0.22; and Z is a number in a range of 0≦Z≦2), and whereinthe sulfide-based solid electrolyte is VLiX—(1−V)((1−W)Li₂S—WP₂S₅)(where X is a halogen element; V is a number in a range of O<V<1; and Wis a number in a range of 0.125≦W≦0.30).

The oxide-based solid electrolyte may be (Li_(7−3Y−Z),Al_(Y))(La₃)(Zr_(2−Z), M_(Z))O₁₂ (where M is at least one elementselected from the group consisting of Nb and Ta; Y is a number in arange of 0≦Y<0.22; and Z is a number in a range of 0<Z≦2).

The sulfide-based solid electrolyte may be at least one selected fromthe group consisting of 0.2LiI—0.8(0.75Li₂S—0.25P₂S₅) and0.2LiBr—0.8(0.75Li₂S—0.25P₂S₅).

The mixing ratio of the sulfide-based solid electrolyte in the compositesolid electrolyte may be 5 volume % or more and 50 volume % or less.

According to the disclosed embodiments, a composite solid electrolytewith excellent formability and chemical stability and high lithium ionconductivity can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example of the composite solidelectrolyte according to an embodiment;

FIG. 2 is a sectional SEM image of a composite solid electrolyteobtained in Example 3;

FIG. 3 is an S element distribution map obtained by EDX of the compositesolid electrolyte of Example 3; and

FIG. 4 is a graph with lithium ion conductivity on the vertical axisand, on the horizontal axis, the volume fraction of a sulfide-basedsolid electrolyte when the total content of the sulfide-based solidelectrolyte and an oxide-based solid electrolyte is determined as 100volume %.

DETAILED DESCRIPTION

The composite solid electrolyte according to the disclosed embodimentsis a composite solid electrolyte comprising an oxide-based solidelectrolyte and a sulfide-based solid electrolyte, wherein theoxide-based solid electrolyte is (Li_(7−3Y−Z), Al_(Y))(La₃)(Zr_(2−Z),M_(Z))O₁₂ (where M is at least one element selected from the groupconsisting of Nb and Ta; Y is a number in a range of 0≦Y<0.22; and Z isa number in a range of 0≦Z≦2), and wherein the sulfide-based solidelectrolyte is VLiX—(1−V)((1−W)Li₂S—WP₂S₅) (where X is a halogenelement; V is a number in a range of 0<V<1; and W is a number in a rangeof 0.125≦W≦0.30).

A sulfide-based solid electrolyte can be easily formed at roomtemperature and has high lithium ion conductivity. However, it mayproduce hydrogen sulfide by exposure to the air.

Meanwhile, an oxide-based solid electrolyte is stable in the air;however, it needs heating at a temperature of close to 1000° C. to beformed.

Therefore, a composite solid electrolyte has been proposed, which isprovided with both high lithium ion conductivity and easy formability atroom temperature, by the use of a highly-flexible sulfide-based solidelectrolyte for the boundaries of the single particles of theoxide-based solid electrolyte.

However, for prior-art composite solid electrolytes, activation energyat the time of charge transfer of an interface between the oxide-basedsolid electrolyte and the sulfide-based solid electrolyte is very high.That is, resistance at the interface is very high. Accordingly, there isa problem in that lithium ion transfer at the interface between theoxide-based solid electrolyte and the sulfide-based solid electrolyte isinhibited, and very low lithium ion conductivity is obtained.

The reason for this is supposed as follows: in the combination of theoxide-based and sulfide-based solid electrolytes used for prior-artcomposite solid electrolytes, a chemical reaction occurs between theoxide-based solid electrolyte and the sulfide-based solid electrolyte toform a high-resistance interface layer.

It was found that higher lithium conductivity than that of prior-artcomposite solid electrolytes can be obtained by use of the sulfide-basedsolid electrolyte containing LiX (where X is a halogen element).

The reason for this is thought as follows: by the use of thesulfide-based solid electrolyte containing LiX, the activation energy atthe time of charge transfer at the interface between the oxide-basedsolid electrolyte and the sulfide-based solid electrolyte is decreased,and a chemical reaction at the interface between the oxide-based solidelectrolyte and the sulfide-based solid electrolyte is less likely tooccur; therefore, the resistance at the interface is decreased, and highlithium ion conductivity is obtained.

The composite solid electrolyte according to the disclosed embodimentshas excellent formability; therefore, a battery can be formed in a roomtemperature or low temperature condition, and battery production becomeseasy.

Also, the composite solid electrolyte according to the disclosedembodiments has excellent chemical stability; therefore, the productionof hydrogen sulfide can be prevented as much as possible.

Also, the composite solid electrolyte according to the disclosedembodiments has high lithium ion conductivity; therefore, a high-powerbattery can be produced.

FIG. 1 is a schematic view of an example of the composite solidelectrolyte according to an embodiment.

As shown in FIG. 1, the composite solid electrolyte that contains anoxide-based solid electrolyte 1 having excellent chemical stability anda sulfide-based solid electrolyte 2 having excellent formability at roomtemperature and containing LiX (where X is a halogen element) at a givenratio, can form a good interface between the oxide-based solidelectrolyte 1 and the sulfide-based solid electrolyte 2, and ion pathsare appropriately obtained; therefore, desired lithium ion conductivitycan be obtained.

The lithium ion conductivity of the composite solid electrolyteaccording to the disclosed embodiments is not particularly limited. Forexample, it may be 1×10⁻⁶ S/cm or more at ordinary temperature.

In the disclosed embodiments, the average particle diameter of particlesis calculated by a general method. An example of the method forcalculating the average particle diameter of particles is as follows.First, for a particle shown in an image taken at an appropriatemagnitude (e.g., 50,000× to 1,000,000×) with a transmission electronmicroscope (hereinafter referred to as TEM) or a scanning electronmicroscope (hereinafter referred to as SEM), the diameter is calculatedon the assumption that the particle is spherical. Such a particlediameter calculation by TEM or SEM observation is carried out on 200 to300 particles of the same type, and the average of the particles isdetermined as the average particle diameter.

[Oxide-Based Solid Electrolyte]

The oxide-based solid electrolyte is not particularly limited, as longas it is (Li_(7−3Y−Z), Al_(Y))(La₃)(Zr_(2−Z), M_(Z))O₁₂ (where M is atleast one element selected from the group consisting of Nb and Ta; Y isa number in a range of 0≦Y<0.22; Z is a number in a range of 0≦Z≦2).From the viewpoint of increasing lithium ion conductivity, Z may be in arange of 0<Z≦2.

As the oxide-based solid electrolyte, examples includeLi_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂, Li₇La₃Zr₂O₁₂,Li_(6.75)La₃Zr_(1.75)Nb_(0.25)O₁₂, Li₅La₃Nb₂O₁₂,(Li_(6.4)Al_(0.2))La₃Zr₂O₁₂ and(Li_(6.15)Al_(0.2))La₃Zr_(1.75)Nb_(0.25)O₁₂. Of them, the oxide-basedsolid electrolyte may be Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂.

The oxide-based solid electrolyte may be in a particle form, forexample. The average particle diameter of the oxide-based solidelectrolyte in a particle form is not particularly limited, and it maybe in a range of from 1 to 10 μm.

[Sulfide-Based Solid Electrolyte]

The sulfide-based solid electrolyte is not particularly limited, as longas it is VLiX—(1−V)((1−W)Li₂S—WP₂S₅) (where X is a halogen element; V isa number in a range of 0<V<1; and W is a number in a range of0.125≦W≦0.30).

With respect to the total of Li₂S and P₂S₅, the ratio of P₂S₅ may be ina range of from 12.5 mol % to 30 mol %; it may be in a range of from 20mol % to 30 mol %; and it may be 25 mol %.

For the Li₂S—P₂S₅-based, sulfide-based solid electrolyte material, it isknown that the material in a crystallized glass state shows high lithiumion conductivity, when the ratio of P₂S₅ is in a range of from 12.5 mol% to 30 mol % with respect to the total of Li₂S and P₂S₅.

A crystallized glass is not a perfect crystal; therefore, it isdifficult to absolutely identify the crystal structure. However, when alithium halide (LiX) is dissolved in the Li₂S—P₂S₅-based, sulfide-basedsolid electrolyte material in the above compositional range, there is atendency of characteristic peaks to appear by X-ray diffraction (XRD)measurement, which are similar to a high lithium conducting phase calledLGPS (Li₁₀GeP₂S₁₂).

From the above facts, it is thought that the lithium halide has aneffect of turning the crystal structure of the sulfide-based solidelectrolyte material into a high lithium conducting phase. Therefore, itis thought that a composite solid electrolyte with high lithium ionconductivity can be obtained by the use of the sulfide-based solidelectrolyte material in the above compositional range.

In LiX, X is a halogen element. As X, examples include F, Cl, Br and I.X may be Cl, Br or I. Also, X may be Br or I. This is because acomposite solid electrolyte with high lithium ion conductivity can beobtained.

The ratio of LiX in the sulfide-based solid electrolyte used in thedisclosed embodiments is not particularly limited. For example, it maybe more than 14 mol % and less than 30 mol %; it may be 15 mol % or moreand 25 mol % or less; and it may be 20 mol %.

As the sulfide-based solid electrolyte, examples include0.2LiBr—0.8(0.75Li₂S—0.25P₂S₅) and 0.2LiI—0.8(0.75Li₂S—0.25P₂S₅).

The sulfide-based solid electrolyte may be in a particle form, forexample. The average particle diameter of the sulfide-based solidelectrolyte in a particle form is not particularly limited, and it maybe in a range of from 0.1 to 10 μm, for example.

The method for producing the sulfide-based solid electrolyte is notparticularly limited. Examples thereof include the following method:first, a raw material composition containing LiX, Li₂S and P₂S₅ isprepared; next, mechanical milling is carried out on the raw materialcomposition, thereby synthesizing a sulfide glass that has LiX and anion conductor having Li, P and S; and the sulfide glass is heated at atemperature equal to or higher than the crystallization temperature,thereby obtaining the sulfide-based solid electrolyte.

[Method for Producing Composite Solid Electrolyte]

The method for producing the composite solid electrolyte according tothe disclosed embodiments is not particularly limited. For example, thecomposite solid electrolyte can be obtained by mixing the oxide-basedsolid electrolyte and the sulfide-based solid electrolyte and compactingthe resulting powder mixture.

The mixing ratio of the sulfide-based solid electrolyte in the compositesolid electrolyte is not particularly limited. From the viewpoint ofincreasing formability and chemical stability, it may be 5 volume % ormore and less than 100 volume %. From the viewpoint of reducing ahydrogen sulfide gas yield, it may be 5 volume % or more and 50 volume %or less. From the viewpoint of obtaining desired lithium ionconductivity, it may be 10 volume % or more and 40 volume % or less.

The mixing method is not particularly limited. When the mixing iscarried out using a mortar, examples include mixing with applying themechanical energy of a ball mill, vibrating mill, turbo mill,mechanofusion, disk mill, etc.

The mixing time is not particularly limited. For example, when themixing is carried out using a vibrating mill, it may be 1 to 60 minutes.

The mixing may be either wet mixing or dry mixing.

EXAMPLES Examples 1 to 5, Comparative Examples 1 and 2 [Synthesis ofOxide-Based Solid Electrolyte]

As an oxide-based solid electrolyte, Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂ wassynthesized.

Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂ was synthesized in a temperature range offrom 500 to 1300° C., using LiOH(H₂O) (manufactured by: Siguma-Aldrich),La(OH)₃ (manufactured by: Kojundo Chemical Laboratory Co., Ltd.), ZrO₂(manufactured by: Kojundo Chemical Laboratory Co., Ltd.) and Nb₂O₅(manufactured by: Kojundo Chemical Laboratory Co., Ltd.) as startingmaterials. It was confirmed by SEM that the average particle diameter ofLi_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂ is about 10 μm.

[Synthesis of Sulfide-Based Solid Electrolyte]

As a sulfide-based solid electrolyte, 0.2LiI—0.8(0.75Li₂S—0.25P₂S₅) wassynthesized.

For the synthesis of the sulfide-based solid electrolyte, lithiumsulfide (Li₂S, manufactured by: Nippon Chemical Industrial Co., Ltd.),diphosphorus pentasulfide (P₂S₅, manufactured by: Aldrich) and lithiumiodide (LiI, manufactured by: Aldrich) were used as starting materials.

Next, Li₂S and P₂S₅ were weighted at a molar ratio of 75Li₂S·25P₂S₅.

Next, LiI was weighed so that the ratio of LiI becomes 20 mol %.

The weighed starting materials were mixed for five minutes with an agatemortar. Then, 2 g of the mixture was put in the container (45 cm³, madeof ZrO₂) of a planetary ball mill. Dehydrated heptane (moisture content30 ppm or less, 4 g) was put in the container. In addition, ZrO₂ balls(diameter 5 mm, 53 g) were put in the container, and the container wasabsolutely hermetically closed. This container was installed in theplanetary ball mill (product name: P7; manufactured by: Fritsch) andmechanical milling was carried out at a plate rotational frequency of500 rpm for 40 hours. Then, the mixture was dried at 100° C. for removalof the heptane, thereby obtaining a sulfide glass.

Then, 0.5 g of the sulfide glass was put in a glass tube. The glass tubewas put in a hermetically-closed container made of SUS. Thehermetically-closed container was heated at 190° C. for 10 hours,thereby obtaining 0.2LiI—0.8(0.75Li₂S—0.25P₂S₅).

[Production of Composite Solid Electrolyte]

Next, Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂ and 0.2LiI—0.8(0.75Li₂S—0.25P₂S₅)were mixed for 30 minutes with a vibrating mill so that the volumefraction of 0.2LiI—0.8(0.75Li₂S—0.25P₂S₅) becomes 0% (ComparativeExample 1), 10% (Example 1), 20% (Example 2), 30% (Example 3), 40%(Example 4), 50% (Example 5) and 100% (Comparative Example 2). Thethus-obtained mixture was put in a metal mold and subjected topowder-compacting at a pressure of 1 ton/cm² (≈98 MPa) at roomtemperature, thereby producing a composite solid electrolyte.

[SEM Image Observation]

Cross-sectional SEM observation of the composite solid electrolyte wascarried out by the following process.

First, a fracture cross-section of the composite solid electrolyte wastreated by a cross sectional polisher (CP) manufactured by JEOL Ltd., atan accelerating voltage of 4 kV for a treatment time of 8 hours, therebyproducing an observational surface. Then, the sectional texture wasobserved with a field emission scanning electron microscope (productname: FE-SEM; manufactured by: Hitachi High-Technologies Corporation),and the element distribution state was checked by energy dispersiveX-ray analysis (EDX).

FIG. 2 shows a sectional SEM image of the composite solid electrolyteobtained in Example 3.

FIG. 3 shows an S element distribution map obtained by EDX of thecomposite solid electrolyte of Example 3.

The S element distribution map in FIG. 3 corresponds to the distributionof the sulfide-based solid electrolyte in the composite solidelectrolyte. From the map in FIG. 3, it is clear that the sulfide-basedsolid electrolyte is mainly present at the boundaries of the particlesof the oxide-based solid electrolyte. Therefore, it is clear that thereare almost no conducting paths between the particles of thesulfide-based solid electrolyte. Also, the S element distribution mapshows no diffusion of the S element into the whole composite solidelectrolyte. Therefore, it is clear that there is not a significantreaction of the sulfide-based solid electrolyte with the oxide-basedsolid electrolyte, and the sulfide-based solid electrolyte is stablypresent.

Example 6

A composite solid electrolyte was produced in the same manner as Example3, except that Li₇La₃Zr₂O₁₂ was used as the oxide-based solidelectrolyte in place of Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂.

Li₇La₃Zr₂O₁₂ was synthesized in a temperature range of from 500 to 1300°C., using LiOH(H₂O) (manufactured by: Siguma-Aldrich), La(OH)₃(manufactured by: Kojundo Chemical Laboratory Co., Ltd.) and ZrO₂(manufactured by: Kojundo Chemical Laboratory Co., Ltd.) as startingmaterials. It was confirmed by SEM that the average particle diameter ofLi₇La₃Zr₂O₁₂ is about 10 μm.

Example 7

A composite solid electrolyte was produced in the same manner as Example3, except that Li_(6.75)La₃Zr_(1.75)Nb_(0.25)O₁₂ was used as theoxide-based solid electrolyte in place ofLi_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂.

Li_(6.75)La₃Zr_(1.75)Nb_(0.25)O₁₂ was synthesized in a temperature rangeof from 500 to 1300° C., using LiOH(H₂O) (manufactured by:Siguma-Aldrich), La(OH)₃ (manufactured by: Kojundo Chemical LaboratoryCo., Ltd.), ZrO₂ (manufactured by: Kojundo Chemical Laboratory Co.,Ltd.) and Nb₂O₅ (manufactured by: Kojundo Chemical Laboratory Co., Ltd.)as starting materials. It was confirmed by SEM that the average particlediameter of Li_(6.75)La₃Zr_(1.75)Nb_(0.25)O₁₂ is about 10 μm.

Example 8

A composite solid electrolyte was produced in the same manner as Example3, except that Li₅La₃Nb₂O₁₂ was used as the oxide-based solidelectrolyte in place of Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂.

Li₅La₃Nb₂O₁₂ was synthesized in a temperature range of from 500 to 1300°C., using LiOH(H₂O) (manufactured by: Siguma-Aldrich), La(OH)₃(manufactured by: Kojundo Chemical Laboratory Co., Ltd.) and Nb₂O₅(manufactured by: Kojundo Chemical Laboratory Co., Ltd.) as startingmaterials. It was confirmed by SEM that the average particle diameter ofLi₅La₃Nb₂O₁₂ is about 10 μm.

Example 9

A composite solid electrolyte was produced in the same manner as Example3, except that (Li_(6.4)Al_(0.2))La₃Zr₂O₁₂ was used as the oxide-basedsolid electrolyte in place of Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂.

(Li_(6.4)Al_(0.2))La₃Zr₂O₁₂ was synthesized in a temperature range offrom 500 to 1300° C., using LiOH(H₂O) (manufactured by: Siguma-Aldrich),γ-Al₂O₃ (manufactured by: Kojundo Chemical Laboratory Co., Ltd.),La(OH)₃ (manufactured by: Kojundo Chemical Laboratory Co., Ltd.) andZrO₂ (manufactured by: Kojundo Chemical Laboratory Co., Ltd.) asstarting materials. It was confirmed by SEM that the average particlediameter of (Li_(6.4)Al_(0.2))La₃Zr₂O₁₂ is about 10 μm.

Example 10

A composite solid electrolyte was produced in the same manner as Example3, except that (Li_(6.15)Al_(0.2))La₃Zr_(1.75)Nb_(0.25)O₁₂ was used asthe oxide-based solid electrolyte in place ofLi_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂.

(Li_(6.15)Al_(0.2))La₃Zr_(1.75)Nb_(0.25)O₁₂ was synthesized in atemperature range of from 500 to 1300° C., using LiOH(H₂O) (manufacturedby: Siguma-Aldrich), γ-Al₂O₃ (manufactured by: Kojundo ChemicalLaboratory Co., Ltd.), La(OH)₃ (manufactured by: Kojundo ChemicalLaboratory Co., Ltd.), ZrO₂ (manufactured by: Kojundo ChemicalLaboratory Co., Ltd.) and Nb₂O₅ (manufactured by: Kojundo ChemicalLaboratory Co., Ltd.) as starting materials. It was confirmed by SEMthat the average particle diameter of(Li_(6.15)Al_(0.2))La₃Zr_(1.75)Nb_(0.25)O₁₂ is about 10 μm.

Example 11

A composite solid electrolyte was produced in the same manner as Example3, except that Li_(6.75)La₃Zr_(1.75)Nb_(0.25)O₁₂ was used as theoxide-based solid electrolyte in place ofLi_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂, and 0.2LiBr—0.8(0.75Li₂S—0.25P₂S₅) wasused as the sulfide-based solid electrolyte in place of0.2LiI—0.8(0.75Li₂S—0.25P₂S₅).

Comparative Example 3

A composite solid electrolyte was produced in the same manner as Example3, except that 0.75Li₂S—0.25P₂S₅ was used as the sulfide-based solidelectrolyte in place of 0.2LiI—0.8(0.75Li₂S—0.25P₂S₅).

Comparative Examples 4 to 10

A composite solid electrolyte was produced in the same manner as Example1, except that Li₇La₃Zr₂O₁₂ was used as the oxide-based solidelectrolyte in place of Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂;0.75Li₂S—0.25P₂S₅ was used as the sulfide-based solid electrolyte inplace of 0.2LiI—0.8(0.75Li₂S—0.25P₂S₅); and Li₇La₃Zr₂O₁₂ and0.75Li₂S—0.25P₂S₅ were mixed so that the volume fraction of0.75Li₂S—0.25P₂S₅ becomes 0% (Comparative Example 4), 12% (ComparativeExample 5), 22% (Comparative Example 6), 26% (Comparative Example 7),31% (Comparative Example 8), 40% (Comparative Example 9) and 100%(Comparative Example 10).

[Lithium Ion Conductivity Measurement]

Lithium ion conductivity measurement was carried out on the compositesolid electrolytes obtained in Examples 1 to 11 and Comparative Examples1 to 10. The measurement was carried out by a AC impedance measurementmethod, using potentiostat 1470 (manufactured by: Solartron) andimpedance analyzer FRA1255B (manufactured by: Solartron) in thefollowing conditions: a voltage swing of 20 mV, a measuring frequency(f) of 0.1 Hz to 1 MHz, a measurement temperature of 25° C. and aconfining pressure of 6 N. Lithium ion conductivities obtained by the ACimpedance measurement are shown in FIG. 4 and Table 1.

FIG. 4 is a graph for the composite solid electrolytes of Examples 1 to5 and Comparative Examples 1 to 10, with lithium ion conductivity (S/cm)on the vertical axis and, on the horizontal axis, the volume fraction(%) of the sulfide-based solid electrolyte when the total content of thesulfide-based solid electrolyte and the oxide-based solid electrolyte isdetermined as 100 volume %.

TABLE 1 Volume Volume fraction fraction (%) (%) Lithium of oxide- ofsulfide- ion Composite of Composite of based based conduc- oxide-basedsulfide-based solid solid tivity solid electrolyte solid electrolyteelectrolyte electrolyte (S/cm) ComparativeLi_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂ 0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 100 0 5.00× 10⁻⁹ Example 1 Example 1 Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 90 10 8.11 × 10⁻⁶ Example 2Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂ 0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 80 20 7.58× 10⁻⁵ Example 3 Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 70 30 2.20 × 10⁻⁴ Example 4Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂ 0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 60 40 3.87× 10⁻⁴ Example 5 Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 50 50 6.47 × 10⁻⁴ ComparativeLi_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂ 0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 0 100 2.60× 10⁻³ Example 2 Example 6 Li₇La₃Zr₂O₁₂ 0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 7030 4.80 × 10⁻⁵ Example 7 Li_(6.75)La₃Zr_(1.75)Nb_(0.25)O₁₂0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 70 30 2.44 × 10⁻⁴ Example 8 Li₅La₃Nb₂O₁₂0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 70 30 5.30 × 10⁻⁵ Example 9(Li_(6.4)Al_(0.2))La₃Zr₂O₁₂ 0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 70 30 1.25 ×10⁻⁵ Example 10 (Li_(6.15)Al_(0.2)) 0.2Lil-0.8(0.75Li₂S-0.25P₂S₅) 70 301.90 × 10⁻⁴ La₃Zr_(1.75)Nb_(0.25)O₁₂ Example 11Li_(6.75)La₃Zr_(1.75)Nb_(0.25)O₁₂ 0.2LiBr-0.8(0.75Li₂S-0.25P₂S₅) 70 301.80 × 10⁻⁴ Comparative Li_(6.4)La₃Zr_(1.4)Nb_(0.6)O₁₂ 0.75Li₂S-0.25P₂S₅70 30 4.00 × 10⁻⁶ Example 3 Comparative Li₇La₃Zr₂O₁₂ 0.75Li₂S-0.25P₂S₅100 0 2.00 × 10⁻⁹ Example 4 Comparative Li₇La₃Zr₂O₁₂ 0.75Li₂S-0.25P₂S₅88 12 1.50 × 10⁻⁷ Example 5 Comparative Li₇La₃Zr₂O₁₂ 0.75Li₂S-0.25P₂S₅78 22 4.00 × 10⁻⁷ Example 6 Comparative Li₇La₃Zr₂O₁₂ 0.75Li₂S-0.25P₂S₅74 26 1.00 × 10⁻⁶ Example 7 Comparative Li₇La₃Zr₂O₁₂ 0.75Li₂S-0.25P₂S₅69 31 2.00 × 10⁻⁶ Example 8 Comparative Li₇La₃Zr₂O₁₂ 0.75Li₂S-0.25P₂S₅60 40 1.00 × 10⁻⁹ Example 9 Comparative Li₇La₃Zr₂O₁₂ 0.75Li₂S-0.25P₂S₅ 0100  5.0 × 10⁻⁴ Example 10

As shown in Table 1, the lithium ion conductivities of the compositesolid electrolytes are as follows: 8.11×10⁻⁶ S/cm in Example 1;7.58×10⁻⁵ S/cm in Example 2; 2.20×10⁻⁴ S/cm in Example 3; 3.87×10⁻⁴ S/cmin Example 4; 6.47×10⁻⁴ S/cm in Example 5; 4.80×10⁻⁵ S/cm in Example 6;2.44×10⁴ S/cm in Example 7; 5.30×10⁻⁵ S/cm in Example 8; 1.25×10⁻⁵ S/cmin Example 9; 1.90×10⁻⁴ S/cm in Example 10; 1.80×10⁻⁴ S/cm in Example11; 5.00×10⁻⁹ S/cm in Comparative Example 1; 2.60×10⁻³ S/cm inComparative Example 2; 4.00×10⁻⁶ S/cm in Comparative Example 3;2.00×10⁻⁹ S/cm in Comparative Example 4; 1.50×10⁻⁷ S/cm in ComparativeExample 5; 4.00×10⁻⁷ S/cm in Comparative Example 6; 1.00×10⁻⁶ S/cm inComparative Example 7; 2.00×10⁻⁶ S/cm in Comparative Example 8;1.00×10⁻⁵ S/cm in Comparative Example 9; and 5.0×10⁻⁴ S/cm inComparative Example 10.

As shown in Table 1 and FIG. 4, it is clear that the composite solidelectrolyte according to the disclosed embodiments, which is such acomposite solid electrolyte that the volume fraction of thesulfide-based solid electrolyte containing the lithium halide is morethan 0% and less than 100%, has higher lithium ion conductivity comparedto a prior-art composite solid electrolyte using a sulfide-based solidelectrolyte having the same volume and not containing a lithium halide.

Also, as shown in Table 1 and FIG. 4, as a result of comparing thelithium ion conductivities of the composite solid electrolytes ofExample 1 and Comparative Example 5, in both of which the volumefraction of the sulfide-based solid electrolyte is about 10%, it isclear that the lithium ion conductivity of Example 1 is 54 times higherthan Comparative Example 5.

As a result of comparing the lithium ion conductivities of the compositesolid electrolytes of Example 2 and Comparative Example 6, in both ofwhich the volume fraction of the sulfide-based solid electrolyte isabout 20%, it is clear that the lithium ion conductivity of Example 2 is190 times higher than Comparative Example 6.

The lithium ion conductivities of the composite solid electrolytes ofExamples 3 and 11, in both of which the volume fraction of thesulfide-based solid electrolyte is about 30%, were compared to thelithium ion conductivities of the composite solid electrolytes ofComparative Examples 3 and 8, in both of which the volume fraction ofthe sulfide-based solid electrolyte is about 30%. Therefore, it is clearthat the lithium ion conductivity of Example 3 is 55 times higher thanComparative Example 3 and 110 times higher than Comparative Example 8.It is also clear that the lithium ion conductivity of Example 11 is 45times higher than Comparative Example 3 and 90 times higher thanComparative Example 8.

As a result of comparing the lithium ion conductivities of the compositesolid electrolytes of Example 4 and Comparative Example 9, in both ofwhich the volume fraction of the sulfide-based solid electrolyte is 40%,it is clear that the lithium ion conductivity of Example 4 is 39 timeshigher than Comparative Example 9.

Therefore, as shown in FIG. 4, the composite solid electrolyte accordingto the disclosed embodiments, in which the volume fraction of thesulfide-based solid electrolyte containing the lithium halide is morethan 5% and less than 50%, is expected to increase lithium ionconductivity 10 or more times higher than a prior-art composite solidelectrolyte using a sulfide-based solid electrolyte having the samevolume and not containing a lithium halide.

Also, the lithium ion conductivities of the composite solid electrolytesof Examples 3 and 11, in both of which the volume fraction of thesulfide-based solid electrolyte is 30%, were compared to the lithium ionconductivity of the composite solid electrolyte of Comparative Example3, in which the volume fraction of the sulfide-based solid electrolyteis 30%. As described above, it is clear that the lithium ionconductivity of Example 3 is 55 times higher than Comparative Example 3,and the lithium ion conductivity of Example 11 is 45 times higher thanComparative Example 3.

Therefore, it is clear that the composite solid electrolyte using thesulfide-based solid electrolyte that contains LiBr and/or LiI, is ableto increase the lithium ion conductivity 45 to 55 times higher than aprior-art composite solid electrolyte using a sulfide-based solidelectrolyte having the same volume and not containing a lithium halide.Therefore, it is supposed that, like the composite solid electrolyteusing the sulfide-based solid electrolyte that contains LiBr and/or LiI,even the composite solid electrolyte using the sulfide-based solidelectrolyte that contains LiF and/or LiCl, can also increase lithium ionconductivity.

As disclosed in paragraph 0060 in the description of WO2012/176808, asfor the lithium ion conductivities of oxide-based solid electrolytes,while the lithium ion conductivity of Li₇La₃Zr₂O₁₂ containing Zr is2.0×10 ⁴ S/cm, the lithium ion conductivity of Li₅La₃Nb₂O₁₂ containingNb is 6.0×10⁻⁵ S/cm. From this fact, it is clear that the lithium ionconductivity of Li₇La₃Zr₂O₁₂ is higher than Li₅La₃Nb₂O₁₂.

However, as shown in Table 1 and FIG. 4, as a result of comparing thelithium ion conductivities of the composite solid electrolytes ofExamples 6 and 8, it is clear that the lithium ion conductivity of thecomposite solid electrolyte of Example 8, in which Li₅La₃Nb₂O₁₂containing Nb is used as the oxide-based solid electrolyte, is higherthan the composite solid electrolyte of Example 6, in which Li₇La₃Zr₂O₁₂containing Zr is used as the oxide-based solid electrolyte.

It is supposed that this is because the interface between the oxide andthe sulfide gets better by containing Nb. However, it is supposed thatwhen the Nb content is too large, the performance of the oxide maydecrease.

1. A composite solid electrolyte comprising an oxide-based solidelectrolyte and a sulfide-based solid electrolyte, wherein theoxide-based solid electrolyte is (Li_(7−3Y−Z), Al_(Y))(La₃)(Zr_(2−Z),M_(Z))O₁₂ (where M is at least one element selected from the groupconsisting of Nb and Ta; Y is a number in a range of 0≦Y<0.22; and Z isa number in a range of 0≦Z≦2), and wherein the sulfide-based solidelectrolyte is VLiX—(1−V)((1−W)Li₂S—WP₂S₅) (where X is a halogenelement; V is a number in a range of 0<V<1; and W is a number in a rangeof 0.125≦W≦0.30).
 2. The composite solid electrolyte according to claim1, wherein the oxide-based solid electrolyte is (Li_(7−3Y−Z),Al_(Y))(La₃)(Zr_(2−Z), M_(Z))O₁₂ (where M is at least one elementselected from the group consisting of Nb and Ta; Y is a number in arange of 0≦Y<0.22; and Z is a number in a range of 0<Z≦2).
 3. Thecomposite solid electrolyte according to claim 1, wherein thesulfide-based solid electrolyte is at least one selected from the groupconsisting of 0.2LiI—0.8(0.75Li₂S—0.25P₂S₅) and0.2LiBr—0.8(0.75Li₂S—0.25P₂S₅).
 4. The composite solid electrolyteaccording to claim 1, wherein a mixing ratio of the sulfide-based solidelectrolyte in the composite solid electrolyte is 5 volume % or more and50 volume % or less.